Thallid
Native to Dominaria. Living fungal creatures bred in the Havenwood Forest by the elf Thelon (who had borrowed some ideas from Thrull creator Endrek Sahr; see Thrull), Thallids were to be used as a food source by the elves when the slowly chilling climate killed off many of their traditional sources in the lead-up to the Ice Age. Initially kept successfully in check, the Thallids eventually grew in numbers too large to control. Their unique ability to increase their population through fungal means - principally the creation of spore clouds - and the incredible rate at which they produced Saprolings meant that after a long and desperate struggle, the elves of Havenwood finally fell to their own creations. Thallids never spread far beyond the continent of Sarpadia, though some do wander Rift-Era Dominaria. Unlike the Saprolings, Thallids have changed little in the intervening millennia and remain a disparate, colorful horde of shambling, fungal mutants.
In the game, Thallids were unique to the Fallen Empires expansion, the only set of cards based on Sarpadia, until the release of the Time Spiral set. "Thallid" itself is not a creature type, and all Thallids instead have the "Fungus" classification. The unique theme of the Thallids was their ability to produce free Saproling creature tokens, typically at the rate of one every three turns, and in some cases to turn these tokens into a resource to spend on other abilities. Unfortunately, the slow speed at which they operate has restricted their value in organised play.
(Thallid, Utopia Mycon)
Read more about this topic: List Of Species In Magic: The Gathering